For several decades nitroglycerin has been administered to humans as a vasodilating agent in the treatment of cardiovascular disease. Recently, it has been shown that nitroglycerin so administered is converted in the body to nitric oxide which is the pharmacologically active metabolite. Still more recently, nitric oxide has been shown to be formed enzymatically from arginine as a normal metabolite which is an important component of endothelium-derived relaxing factors (EDRFs). EDRFs are currently being intensively studied as participating in regulation of blood flow and vascular resistance. In addition to vascular endothelium, macrophages have also been shown to produce nitric oxide in the body which is a component of their cell killing and/or cytostatic function.
It has been established that the enzyme forming nitric oxide from arginine, i.e., nitric oxide synthase, occurs in two distinct types, namely the constitutive forms and an inducible form. Constitutive forms are present in normal endothelial cells, neurons and some other tissues. Formation of nitric oxide by constitutive form in endothelial cells is thought to play a role in normal blood pressure regulation. The inducible form of nitric oxide synthase has been found to be present in activated macrophages and is induced in endothelial cells and vascular smooth muscle cells, for example, by various cytokines and/or microbial products. It is thought that in sepsis or cytokine-induced shock, overproduction of nitric oxide by the inducible form of nitric oxide synthase plays an important role in the observed life-threatening hypotension. Furthermore, it is thought that overproduction of nitric oxide by the inducible form of nitric oxide synthase is a basis for insensitivity to pressor agents such as .alpha..sub.1 -adrenergic agonists used in the treatment of septic or cytokine-induced shock patients.
Considerable research effort has been expended to discover inhibitors for nitric oxide synthase activity. In large measure, said research effort has been directed at uncovering arginine analogs which inhibit nitric oxide synthase activity by blocking the use of arginine as substrate for nitric oxide synthase.